The present invention relates to a calcium-magnesium carbonate composite and a method for the preparation thereof. More particularly, the invention relates to a calcium-magnesium carbonate composite having excellent whiteness and a reinforcing effect on plastics and useful as a pigment in paper coating capable of giving excellent characteristics to the coated paper as well as a filler in polymeric materials.
As is well known, calcium carbonate powders have excellent whiteness and other desirable properties so that they are consumed in large quantities as a filler or pigment in a variety of fields including the industries of paper, rubbers, plastics, paints and the like. Particles of calcium carbonate have a crystalline structure of calcite giving cubic or spindle-like particles, aragonite giving columnar or acicular particles, vaterite giving spherical particles or the like. It is usual that a calcium carbonate powder is prepared by the carbonation of calcium hydroxide in an aqueous slurry or a lime milk using carbon dioxide gas or a carbon dioxide-containing gas blown into the slurry.
Though excellent in respect of the whiteness and absorptivity of printing inks when used as a pigment for paper coating as compared with kaolin clay consisting of platelet particles, conventional calcium carbonate fillers have a serious defect that the paper coated by using the same is generally poor in the white-paper gloss, printing gloss, printing strength and the like. Attempts and proposals have been made for the preparation and application of a calcium carbonate filler predominantly consisting of plate-like or flaky particles, with which the above mentioned problems could be solved, but no such a product is commercially available because of the high costs for the production of such a calcium carbonate powder.
Carbonates of magnesium, such as magnesium carbonate hydroxide, i.e. basic magnesium carbonate, are also widely employed as an inorganic filler in various polymeric products including rubbers and plastics or, in particular, as a filler for transparency formulation of natural rubbers. Magnesium carbonates in general have a plate-like or flaky particle configuration to exhibit a reinforcing effect as a filler in plastics. They are also used as a white filler or pigment in the applications for paints, cosmetic compositions, paper coating and the like as well as carrier of medicines and perfumes. Magnesium carbonates, however, have a defect that the reproducibility of coloring by use thereof is poor when they are used in combination with other pigments or fillers or, in particular, natural talc.
Several methods are known and practiced for the preparation of a magnesium carbonate hydroxide filler including carbonation of magnesium hydroxide, alkali carbonation of bittern or magnesium chloride, thermal decomposition of magnesium hydrogencarbonate and so on. When the method is performed by a wet process in an aqueous suspension or slurry, however, a difficulty is encountered that the slurry sometimes has an unduly high consistency due to the fine platelet-like particle configuration of the particles which also leads to a trouble that the particles are readily converted into a hard cake which must be dried with some difficulties and subsequently disintegrated into a fine powder before the reaction product is practically used.
Conventional magnesium carbonate hydroxide fillers usually have a bulk density so low that difficulties are encountered in the compounding work of the same with a synthetic resin into a uniform blend. Proposals have been made to solve this problem in Japanese patent Kokai No. 60-54915, No. 51-63526, No. 63-89416 and No. 1 224218 according to which spherical secondary particles are formed from the fine plate-like primary particles of the magnesium carbonate hydroxide.
It is a general problem or disadvantage in the above described prior art methods for the preparation of magnesium carbonate hydroxide that the first step of the process is preparation of magnesium orthocarbonate which must be converted into the desired product by aging in a liquid medium at a high temperature. Accordingly, the process :s necessarily very complicated because adequate control is essential In both of the steps for the formation of the magnesium orthocarbonate as a precursor and for the conversion of the same into the magnesium carbonate hydroxide.
As a process for the preparation of a calcium-magnesium carbonate composite, it is known since old that dolomite, which is a double carbonate of calcium and magnesium, typically, of the formula CaMg[CO.sub.3 ].sub.2, is calcined and slaked into a hydroxide composite which is then carbonated into the carbonate composite (see, for example, Japanese patent publications No. 31-7277, No. 32-632, No. 33-1863 and No. 37-4103). In the carbonation reaction of the hydroxide composite, it is known that the reaction proceeds in two steps due to the difference in the basicity of the hydroxides that the carbonation of calcium hydroxide first proceeds followed by the carbonation of magnesium hydroxide so that adequate control of the conditions of the carbonation reaction can provide a uniform composite of calcium carbonate and magnesium carbonate hydroxide. In this case also, however, the magnesium carbonate hydroxide can be formed only from the orthocarbonate as a precursor.